Frost dates for 59275Westby, MT
The average last spring frost in Westby is May 18 and the average first fall frost is September 21, giving a typical growing season of about 125 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- May 18
- Average first fall frost
- Sep 21
- Growing season
- 125 days
Safer after May 30 (1-in-10 risk)
Possible by Sep 7 (1-in-10 risk)
Last frost typically in 203 days
Frost risk by date
NOAA reports frost dates as probabilities. The middle date is the average (a 50% chance). For spring, the 10% date is a safer planting date: frost after it happens in about 1 year in 10. Light frost (36°F) can damage tender plants; a hard freeze (28°F) kills most annuals.
| Temperature | Last spring: 90% / 50% / 10% chance | First fall: 10% / 50% / 90% chance | Growing season (50%) | Years with at least one |
|---|---|---|---|---|
| Light frost (36°F) | May 15 · May 27 · Jun 9 | Aug 31 · Sep 12 · Sep 24 | 106 days | 100% |
| Frost / freeze (32°F) | May 6 · May 18 · May 30 | Sep 7 · Sep 21 · Oct 1 | 125 days | 100% |
| Hard freeze (28°F) | Apr 26 · May 8 · May 20 | Sep 16 · Sep 28 · Oct 11 | 141 days | 100% |
Planting calendar for Westby
Planting windows from the University of Maryland Extension calendar, shifted to your average frost dates. Use them as a starting point and adjust for seed packet instructions and local conditions.
| Crop | Task | Window |
|---|---|---|
| Tomatoes | Start seeds indoors | Apr 17 – Jun 23 |
| Tomatoes | Transplant outdoors | Jun 3 – Jun 1 |
| Peppers | Start seeds indoors | Apr 17 – May 18 |
| Peppers | Transplant outdoors | Jun 17 – Jul 18 |
| Eggplant | Start seeds indoors | May 4 – Jun 2 |
| Eggplant | Transplant outdoors | Jun 17 – Jul 18 |
| Cucumbers | Sow outdoors | Jun 17 – Jun 11 |
| Summer squash | Sow outdoors | Jun 3 – Jul 18 |
| Winter squash & pumpkins | Sow outdoors | Jun 17 – Jul 18 |
| Snap beans | Sow outdoors | Jun 3 – Jul 4 |
| Sweet corn | Sow outdoors | Jun 3 – Jun 1 |
| Watermelon & cantaloupe | Sow outdoors | Jun 17 – Jul 18 |
| Okra | Sow outdoors | Jun 10 – Jul 3 |
| Peas | Sow outdoors (spring) | Apr 17 – Jun 2 |
| Potatoes | Plant outdoors (spring) | Apr 17 – Jun 17 |
| Onions | Start seeds indoors (spring) | Mar 6 – Apr 2 |
| Onions | Transplant outdoors (spring) | May 4 – Jun 2 |
| Broccoli | Start seeds indoors (spring) | Mar 30 – May 3 |
| Broccoli | Transplant outdoors (spring) | May 4 – Jun 2 |
| Broccoli | Start seeds indoors (fall) | May 22 – Jun 1 |
| Broccoli | Transplant outdoors (fall) | Jun 21 – Jul 2 |
| Cabbage | Start seeds indoors (spring) | Mar 20 – Apr 17 |
| Cabbage | Transplant outdoors (spring) | Apr 17 – May 18 |
| Cabbage | Start seeds indoors (fall) | May 12 – Jun 11 |
| Cabbage | Transplant outdoors (fall) | Jun 11 – Jul 12 |
| Leaf lettuce | Sow outdoors (spring) | Apr 17 – Jul 3 |
| Leaf lettuce | Sow outdoors (fall) | Jun 11 – Jul 28 |
| Spinach | Sow outdoors (spring) | Apr 10 – May 24 |
| Spinach | Sow outdoors (fall) | Jun 28 – Aug 2 |
| Radishes | Sow outdoors (spring) | Apr 17 – Jun 7 |
| Radishes | Sow outdoors (fall) | Jun 11 – Aug 12 |
| Kale | Sow outdoors (spring) | May 4 – May 24 |
| Kale | Sow outdoors (fall) | Jun 4 – Jul 12 |
| Carrots | Sow outdoors (spring) | May 18 – Jul 3 |
| Carrots | Sow outdoors (fall) | May 12 – Jun 27 |
| Garlic | Plant outdoors (fall) | Sep 11 – Oct 12 |
Where these dates come from
ZIP code 59275 is matched to the NOAA station Westby, MT, 5.7 km away, at 2,103 ft elevation (59 ft lower than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Westby, MT (used) | 2,103 ft | May 18 | Sep 21 |
| Fortuna 1 W, ND | 2,350 ft | May 19 | Sep 17 |
| Plentywood, MT | 2,048 ft | May 25 | Sep 14 |
| Raymond Border Stn, MT | 2,345 ft | May 19 | Sep 23 |
Frost dates near Westby
- Fortuna, ND 58844 · last May 19, first Sep 17
- Dagmar, MT 59219 · last May 16, first Sep 18
- Antelope, MT 59211 · last May 25, first Sep 14
- Plentywood, MT 59254 · last May 25, first Sep 14
- Raymond, MT 59256 · last May 19, first Sep 23
- Grenora, ND 58845 · last May 18, first Sep 21
- Outlook, MT 59252 · last May 19, first Sep 23
- Zahl, ND 58856 · last May 18, first Sep 20
Frequently asked questions
- When is the last frost in Westby, MT?
- For ZIP code 59275, the average last spring frost (32°F) is May 18, based on NOAA 1991-2020 climate normals for the Westby, MT weather station. There is a 1-in-10 chance of a frost after May 30.
- When is the first frost in Westby, MT?
- The average first fall frost (32°F) is September 21. There is a 1-in-10 chance of frost by September 7, so tender plants are at risk from that point on.
- How long is the growing season in 59275?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 125 days long.
- Are these frost dates a forecast?
- No. They are 30-year averages and probabilities from NOAA weather station records. Use them to plan, then watch your local forecast before planting tender crops or leaving them uncovered.